Can a level 9 player join a Giant mushroom battle?
No. A Giant mushroom needs a squad of 20, but a level 9 player can only field 10 Pikmin. You'll be able to join once you reach level 12.
Your maximum squad size grows with your level, and each mushroom size has a minimum squad you must field to start a battle. If your cap is below that minimum, you can't join β no matter how strong your Pikmin are.